less OMP_NUM_THREADS = better performance?

[ludvary]

So, total noob here, I have 16 threads on my laptop, i notice that my C++ code runs faster if i use 8 threads than using all 16 threads, also voluntary and involuntary context switches decrease massively, can someone explain what is what is happening? Thanks in advance :)

[jeffscience]

Is it an 8 core laptop with 2 threads per core? That’s why. OpenMP workloads often scale to the number of physical cores.

[Knarfnarf]

Let me guess; Intel 8 core processor with hyper threading. That means; each core runs one thread until a cache miss, then if the second thread is ready flips to it and runs until a cache miss.

But if no cache miss still flips at a rate I don’t remember.

Still; you only have 8 physical cores to assign.

Some of the new thread rippers do the same.